Electrochemistry of phenyl-N-capped aniline oligomers. Evaluation of optical and electrochemical properties of ideal polyaniline

Potentials for the first reversible oxidation and optical bandgaps of phenyl-N-capped aniline oligomers (n = 1,2 and 4) are linearly related with 1/n. The comparison of the values extrapolated for the infinitely long polymer and those of electrochemically synthesized polymer samples indicates for the latter an average equivalent length of 6–10 monomeric units.     

Synthesis and characterization of low molecular weight oligomers of soluble polyaniline by electrospray ionization mass spectrometry

Mass spectrometry (MS) has not been utilized extensively for its ability to analyze the structure of polyaniline. This has been a result of polyaniline not being directly soluble in MS-“friendly” solvents. Here, low molecular weight oligomers of polyaniline are synthesized, using a modified room temperature synthesis of polyaniline, that are soluble in solvents that are better suited to electrospray MS analysis. Oligomers up to the 9-mer are identified from the mass spectra. Importantly, several different types of polyaniline oligomers that differ from one another based on their end groups were distinguished in the mass spectra. The use of tandem mass spectrometry (MS/MS) enabled identification of these various end groups.     

Conformations of aniline black (polyaniline) molecules

Conformations of aniline black molecules were studied using a semi-empirical computer-aided analysis. A strong deformation of the ‘planar structure’ of the molecules has been found.     

Retention of polyaniline on glass functionalized with aniline

A novel method is described for the retention of polyaniline onto ordinary glass surfaces. Polyaniline films thus obtained have a uniform texture and good quality. The conductivity (measured in terms of sheet resistance) of the films depends on the concentration ratio between aniline and the oxidant used in the preparation of the polymer. Supporting experiments suggest that the interaction between the polymer and the glass plate is most likely of covalent nature.     

Preparation and characterization of trifluoromethylaniline oligomers

The oxidation of m-trifluoromethylaniline in the presence of trifluoroacetic acid leads to the preparation of poly(m-trifluoromethylaniline). The dimer and a mixture of higher oligomers have been prepared using both cyclic voltammetry for the preparation of thin films, and controlled potential electrooxidation for the preparation of powders. The CF₃ moiety attached to the backbone shifts the oxidation potential of the thin film toward more positive potentials as might be expected because of the electron-withdrawing effects of the CF₃ group. The conductivity of the film has been estimated to be on the order of 10⁻⁸ S/cm when electrooxidized under the appropriate conditions of pH and potential. Extensive analysis of the products of electrooxidation indicates that the solution consists of oxidized monomer, while the dimer and higher molecular weight oligomers precipitate. The dimer can be separated from the other oligomers by vacuum sublimation. Gel permeation chromatography of the remaining precipitate indicates that the precipitate consists of 20% trimer and tetramer, 40% pentamer and hexamer, and about 40% higher molecular weight oligomers.     

EPR and electrical conductivity in microporous polyaniline

We have studied a series of microporous polyaniline (PANI) samples, each of the structure with micro-channels and micro-caves inside (a diameter of 1.5–2.5 μm), formed as a non-periodic network of microrods. Microporous polyaniline is a paramagnetic material and strong EPR signals with the g-factor in the range of 2.0030–2.0032 and the line width ΔB between 1.80 and 6.50 Gs have been measured for samples of various electrical conductivity. The EPR line asymmetry is high (almost 1.5) in the case of PANI samples of the highest electrical conductivity (0.35 S/cm).We found that lithium cations present in the reaction medium can stabilise values of the EPR line asymmetry parameter and g-factor observed for PANI samples prepared in such conditions. Lithium cations are able to influence the intermolecular hydrogen bonding which is essential for the interchain coupling leading to an increase in the EPR line asymmetry and a higher electrical conductivity. Thus, the properties of PANI samples are modified by lithium cations.     

Spectroscopic and thermal properties of the copolymer of aniline with dithiodianiline

Electrochemical copolymerization of aniline (ANI) with dithiodianiline (DTDA), an aniline derivative containing –S–S– links was carried out in 2 M H₂SO₄ using cyclic voltammetry. The cyclicvoltamogram (CV) of the deposited copolymer was recorded in monomer free background electrolyte. A close comparison of the CV characteristics of the films deposited during polymerization of mixture of ANI and DTDA clearly the copolymer deposition. The UV–VIS spectroscopy, spectroelectrochemical studies, thermal analysis and FTIR spectroscopic analysis were used to identify the differences between the copolymer and PANI in the optical, thermal properties and structure. The copolymer was found to have a branched structure in comparison with PANI. The copolymer exhibit different colors upon changing applied potentials.     

Synthesis and characterization of water-soluble polyaniline films

Water-soluble polyaniline (PANI) films were synthesized by using poly(2-acrylamido-2-methyl propanesulphonic acid) (PAMPS) as a water-soluble dopant. The aqueous solution conductivities of PANI/PAMPS films were in the range of 10^?1 to 10⁰ mS/cm which were increased compared with PAMPS films aqueous solution. The structure and microstructure characteristics of PANI/PAMPS films which varied greatly with different molar ratios of aniline/AMPS were investigated by SEM, UV–vis absorption spectroscopy and XRD analysis. Moreover, the photoluminescence properties of PANI/PAMPS films were studied, and water-soluble PANI having appropriate size fitted to the increase of fluorescence emission intensity. PANI/PAMPS films were also investigated by FTIR spectroscopy and TG analysis.     

Morphology characterization of polyaniline nano- and microstructures

Small-angle neutron scattering (SANS), nuclear magnetic resonance (NMR), wide-angle and small-angle X-ray scattering (WAXS and SAXS) measurements were carried out to investigate the three morphological forms of polyaniline emeraldine base (PANI-EB): unstructured, microtubes, and nanofibers. Although the chemical backbone between these two materials is quite similar, their solid structures are quite different, showing differences in the molecular conformation and supramolecular packing. Detailed solid-state ¹³C and ¹⁵N NMR characterization of PANI nanofibers (compared to the unstructured, granular form) revealed a slight variation in the structural features of the polymer that led to some differences in the chemical environments of the respective nuclei. The presence of two extra-sharp peaks at 96.5 and 179.8 ppm is a distinct feature found exclusively in the nanofiber spectra. Moreover, the crosspolarization (CP) dynamics study disclosed the presence of a complete set of sharp NMR peaks that are responsible for the presence of a more ordered morphology in the nanofiber. Small-angle neutron scattering indicated very sharp interfaces in the PANI fibers, which are well organized and have extremely sharp domains within the length scales probed (∼10–1 nm). Overall, the X-ray scattering and spectroscopy data suggest that the nanofiber form is structurally different from the unstructured, PANI-EB powder. These differences are manifested, in part, by the additional chemistry occurring during the synthesis of the nanofibers.     

Synthesis,characterization and dc electrical conductivity of some oligmer mixed metal complexes

The synthesis of the oligomer metal complexes was systematically carried out in three steps starting from condensing terephthaldehyde with 2,5-diamino-3,4-dicyanothiophene in DMF. Dithienylenephenylene (TBT) produced was complexed with copper acetate to give di-TBT copper complex [Cu(TBT)₂]. This was complexed once more with a metal salt to give the 12-ring tri-metal oligomer M[Cu(TBT)₂]₂, where M is Co, Ni, or Cu.The presence of different transition metal ions allowed the comparison of the chemical and physical properties of the mixed metal complexes. The 12-ring mixed metal complexes were characterized through a comparative study of FTIR, electronic spectra and thermal analyses (TGA and DSC) in addition to the elemental analyses and metal content. A comparative study of the dc electrical conductivity in its intrinsic and 5% I₂-doped states with temperature variation in the range 300–500 K allowed tracing the action of the three different transition metals on the dc electrical conductivity of the mixed metal complexes.     

Optical and electrical transport properties of polyaniline–silver nanocomposite

Polyaniline–silver nanocomposite has been synthesized successfully by the chemical oxidative polymerization of aniline with ammonium peroxydisulphate as an initiator in presence of negatively charged silver nanoparticles. Silver nanoparticles are prepared by standard citrate reduction method. TEM, SEM, XRD, FTIR, TGA, DSC, optical absorption and photoluminescence studies are done for the morphological, structural, thermal and optical characterization of the polyaniline nanocomposite. From the TEM and SEM image, it is observed that nanoparticles are well dispersed in the polyaniline matrix. XRD pattern shows that polyaniline is amorphous, but peaks present in XRD pattern in polymer nanocomposites are for silver nanoparticles. TGA and DSC results show that polyaniline silver nanocomposite is more crystalline and more thermally stable. A surface plasmon absorption band is obtained from the optical absorption at 380 nm, which indicates that silver nanoparticles are present in the polyaniline matrix. The optical band gap of nanocomposite decreases with increasing content of silver nanoparticles. An enhancement in photoluminescence has been observed in polyaniline–silver nanocomposite than that in pure polyaniline. The electrical conductivity of polyaniline–silver nanocomposite increases with increase in silver nanoparticle content than that of pure polyaniline. This is a simple way by which optical and electrical properties of polyaniline may be enhanced by doping with suitable nanoparticles.     

Comparison of chiral polyaniline carbon nanotube nanocomposites synthesized by aniline dimer-assisted chemistry and electrochemistry methods

We report herein aniline dimer-assisted chemistry and electrochemistry methods for synthesis of chiral polyaniline carbon nanotube composites. The negative charged carbon nanotubes were first dispersed into aqueous solution, and then aniline monomer, chiral inducing agent camphorsulfonic acid (CSA) and aniline dimer were added into above solution in sequence. Polymerization was initiated by adding oxidizing agent ammonium persulfate solution (chemistry method) or by inserting electrode with appropriate potential (electrochemistry method). Roles of CSA and aniline dimer in determining optical activity of the resulting polyaniline carbon nanotube nanocomposites were investigated in detail in this study. Effects of the carbon nanotube loading and the temperature on the optical activity of the resulting polyaniline carbon nanotube nanocomposites were also investigated. Two different methods (solution chemistry and electrochemistry) were compared in the synthesis of the chiral polyaniline carbon nanotube nanocomposites in this study.     

Morphology and electrical properties of hybrid and sulphonated oxalic acid-doped polyaniline

Polyaniline–polyethylene glycol2000 (PAni–PEG2000) hybrid and sulphonated polyaniline (SPAni) were prepared using oxalic acid as dopant and potassium permanganate as oxidant. The properties of these two conductive polymers were studied in comparison with pure polyaniline (PAni) prepared using the same conditions. The investigated polymers were characterized using FTIR, UV–vis, TGA, TEM, SEM, XRD and their electrical conductivities were also investigated. The morphology of pure polymer was found to be flat ribbon-like form, while presence of polyethylene glycol led to the formation of separate nanospheres. The three polymer samples have different degrees of crystallinity, the highest degree is for SPAni. Unexpectedly, electrical conductivity is in the order SPAni > PAni–PEG2000 > PAni, aging of PAni increases gradually the conductivity of the polymer from 0.0056 to 0.023 S cm^?1 and the increase of temperature gradually decreases the conductivity.     

Polymerization of substituted aniline and characterization of the polymers obtained

Chemical polymerization of m-toluidine, o-chloroaniline and nitroaniline has been studied, and characterization of the resulting polymers performed by DSC, X-ray, i.r., XPS and conductivity measurements. It is found that the substituted group of aniline affects not only the polymerization reaction, but also the properties of the polymers obtained. The polymerization conditions, such as the pH value of the polymerization solution, have a certain influence on the polymer yield and its conductivity. The solubility of polymers of substituted aniline (SPA_n) is similar to that of polyaniline. The results on X-ray diffraction of SPA_n reveal that the substituted group has some effect on the regularity of the polymers. DSC measurements show the existence of an endothermic peak as a disordering of the regular segments. The spin concentration of SPA_n increases with increasing conductivity of the polymers to a certain degree. On the basis of the XPS and doping reaction of SPA_n, a possible structure of the conducting polymer is proposed.     

Potentiodynamic deposition of polyaniline on non-platinum metals and characterization

Polyaniline (PANI) was potentiodynamically deposited on stainless steel (SS), Ni, Ti, Al and Pb electrodes from aqueous solutions of NaClO₄, oxalic acid and H₂SO₄ of different concentrations. Platinum was also used as a substrate for the purpose of comparison. Although, the non-platinum metals showed reactivity in the electrolytes, the adsorption of aniline monomer occurred leading to initiation and growth of PANI during repeated potential sweeps. The nature of voltammograms of the non-platinum metals during PANI deposition differed from that of the Pt electrode. Subsequent to potentiodynamic deposition, the electrodes were studied for their electrochemical activity in 0.5 M H₂SO₄ without and with dissolved redox species. On all metals, the PANI exhibited the potentiodynamic peak at about 0.2 V corresponding to transition from leucoemaraldine (LE) state to emeraldine (EM) state at low scan rates. However, this peak disappeared on repetition of potential scan and also at high scan rates. The PANI deposited on non-platinum metals showed response to dissolved redox species, viz. hydroquinone/quinone, ferrous/ferric, ferrocyanide/ferricyanide, as reflected in the potentiodynamic and ac impedance measurements. However, the peak potential separation of cyclic voltammograms was very large.     

Fabrication and characterization of polyaniline/porous silicon heterojunction

Heterojunction between polyaniline (PANI) and porous silicon (PS) was fabricated by making a layer of PANI on PS, using spin coating method. PS was fabricated by electrochemical etching process. PS was characterized by photoluminescence (PL), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) while the PANI was characterized by FTIR and absorption (UV–VIS) spectroscopy. Current–voltage and capacitance–voltage measurements were done to determine the electrical properties of the heterojunction structure. The ideality factor of the heterojunction was found to be 4.2, which was considered high due to large defect density at the interface. Built-in potential was measured by both I–V and C–V and was found to be Φ_b(I–V) = 0.41 V and Φ_b(C–V) = 0.28 V respectively. The discrepancy in the values of the built-in potential was discussed. Band discontinuity in conduction band and valence band were found to be 0.65 and 1.27 eV respectively. Solar response of the heterojunction was also observed at AM (air mass) 1.0 and it showed a promising behavior as a photovoltaic device.     

Composites of polyaniline and cellulose acetate: preparation,characterization, thermo-oxidative degradation and stability in terms of DC electrical conductivity retention

The composite films of polyaniline and cellulose acetate have been prepared by using two slightly different techniques. Thus-prepared polyaniline:celluloseacetate (PANI:CA) composite films were characterized by FTIR and SEM. The thermo-oxidative degradation was studied by thermogravimetric analysis (TGA) while the stability in terms of retention of DC electrical properties was studied in an oxidative environment by two slightly different techniques.     

Synthesis of sulfonated polyaniline by polymerization of the aniline heterodimer 4-aminodiphenylamine-2-sulfonic acid

The aniline heterodimer 4-aminodiphenylamine-2-sulfonic acid has been polymerized in aqueous ammonium persulfate solution to yield a new form of sulfonated polyaniline (SPAn). This polymer, obtained as a green powder, is soluble in aqueous base (violet solution) and polar solvents (green solution, turning blue on standing). Its FTIR, UV–VIS and EPR spectra are quite similar to those of SPAn obtained by direct ring-sulfonation of polyaniline. The room temperature dc conductivity of the polymer is about 1.3×10⁻⁵ S/cm, indicative of considerable disorder in the sample. Cyclic voltammetry in 1.0 M HCl shows two redox peaks at E_1/2,1=0.50 V and E_1/2,2=0.68 V against Ag/AgCl electrode, characteristic of SPAn.     

The study of electrical and magnetic properties of LiPF6 doped polyaniline

The results of temperature dependent dc conductivity, EPR magnetic susceptibility, and X-ray photoelectron spectroscopy experiments (XPS) are reported for polyaniline (PAN) doped with lithium hexafluorophosphate (LiPF₆), PAN-LiPF₆. DC conductivity (σ_dc) and its temperature dependence of PAN-LiPF₆ samples vary with the average molecular weight (Mw) of polyaniline used and the composition of doping solution. The EPR linewidth and Pauli susceptibility (χ^P) depend on the Mw of PAN used. The σ_dc and the χ^P of the intermediate Mw PAN-LiPF₆ samples are higher than those of the low Mw PAN samples, indicating more highly conducting state of the intermediate Mw PAN samples. Based on the results of XPS and EPR experiments, some portions of quinoid rings in emeraldine base form of PAN are transformed into benzenoid rings through LiPF₆ doping process, indicating the formation of polarons. However, the existence of some portions of quinoid rings in PAN-LiPF₆ samples after doping suggests an inhomogeneous doping or a pseudo doping.